Seismic surveying



July 20, 1943. w BORN 2,324,816

SEISMIC SURVEYING Original Filed May 26, 1933 /2 mg 75 7'9 g 20 55 Li:)L 17 5/ /;5 AMPLIFIER RECORDER VAPOR DISCHARGE SEISMOMETER SHO TINVENTOR ATTORNEYS atented July 20, 1943 NITED STATES PATENT OFFICE ISEISMIC SURVEYING William T. Born, Tulsa, Okla., minor to GeophysicalResearch Corporation,

New York,

N. Y., a corporation of New Jersey original application May 28, 1933,Serial No.

Divided and this application August 19, 1942, Serial No. 455,331

18 Claims. (Cl. 177--352) This invention relates to seismic surveying.

In making seismic surveys by the so-called reflection method, a recordis made of the earth's disturbance produced at a given oint on theearths surface as the result of the detonation of an explosive chargeplaced in the earth's surface at a distant point. In general, the recordshows waves which have traversed paths close to the earths surface andwaves which have penetrated the earth and have been reflected byinterfaces between two layers of different properties orcharacteristics. In many cases, several interfaces are present atvarying depths and the record will show waves reflected from suchinterfaces. In general, the amplitude of the direct waves or wavestraversing a path close to the surface is much greater than theamplitude of the reflected waves and in general, the amplitude of thereflected waves decreases with increase in depth of the interfaces fromwhich they are reflected. Thus, a wave reflected from a hallow interfaceis received earlier and is of greater amplitude than a wave reflectedfrom a deep interface. If the sensitivity of the recording system andthe size of the dynamite charge are so adjusted as to record thereflected waves from a deep interface with the desired amplitude, thenthe amplitude of reflected waves from shallower interfaces will be toolarge for satisfactory recording and the amplitude of the direct waveswill be so great that the characteristics are not readily observablefrom the record produced. In general, the amplitude of the various wavesreceived at the recording position is inversely pro-' portional to thedistance they have traveled through the earth. While the velocities ofthe various waves are not generally the same, they are sufficientlyalike so that the amplitude of the waves received is roughly in inverseproportion to their travel time from the shot point to the recordingposition.

In Weatherby, et al. U. S. Patent No. 2,321,341, issued June 8, 1943, ofwhich this patent is a division, there is disclosed and claimed a waverecording system in which the waves reflected from the interfaces ofdifierent depths are recorded with approximately the same amplitude andthe direct waves are recorded with their recording system, theamplification of the system,

is automatically increased as a function of time so that theearlier-received waves are amplified less than/the later-received waves.The maxibe so adjusted that all the received waves are recorded withtheir amplitudes suiiiciently alike to afford satisfactory observationof all the waves. The system of said application is of such design thatthe increase in amplification of the recording system is initiated atthe instant of detonation of the explosive charge. In some cases, it isdesirable to have the amplification increase begin at the instant thefirst direct wave arrives at the recording position ratherthan to havethe zlrlnphfication increase begin at the time of the An object of thisinvention is a modification of the wave recording system above referredto in which the amplification increase is initiated at the time ofarrival of the first direct wave at the recording position.

Other objects, novel features and advantages of this invention willbecome apparent from the following specification and accompanyingdrawing, wherein the single figure illustrates an embodiment of theinvention.

In the drawing, it indicates an electrical seismometer preferably of theGalitzen type which is located in the earth at the wave-receivingposltion. The seismometer i0, together with the variable section of avoltage divider ii is arranged in the input circuit of a vacuum tube l2which is provided with the usual output circuit. Two resistances I3 andit are connected in series with each other and in parallel with thevoltage divider ii as well as in parallel with a battery l5 through thecontacts of a relay i6 and a switch ii. A condenser i8 is also connectedin parallel with the voltage divider i i. The output circuit of thevacuum tube i2 is inductively connected by the transformer I! to theinput of an amplifier 20 which is connected in turn to a recorder 2i.Alternatively, the tube i2 may constitute the first stage of anamplifier unit.

, current to flow through the voltage divider ii and the resistances i3and I I. The grid potential of the vacuum tube i2 is thus made negativewith respect to the cathode, the absolute value of the grid potentialbeing dependent upon the voltage of the battery i5 and the position ofthe sliding contact of the voltage divider Ii. If then either the switchii or the relay i8 contacts are opened, the condenser it will dischargethrough mum and minimum values of amplification as the resistances inparallel with it, the rate of condenser is completely discharged. As theoutput of the vacuum tube 12 is dependent upon the value of the gridpotential, it will, therefore,

vary as the condenser discharges, being at its minimum when thecondenser is fully charged and at its maximum when the condenser isfully discharged. The ratio of the maximum to minimum amplification maybe adjusted by means of the voltage divider ll while the rate ofdischarge of the condenser l8 may be varied by changing the value of theresistance II. The resistance l3 serves to prevent short-circuiting thebattery l when the resistance I4 is reduced to zero.

At the shot position, a loop 22 of wire is wrapped around the charge ofexplosive and is connected in series with a switch 23, a battery 24 andthe primary 23 of a transformer. The secondary 26 of the transformer isconnected between the control electrode and cathode of a grid-controlledvapor discharge tube rectifier 21 and the output of said rectifier isconnected to a transmission line 23, this line having in one sidethereof, a battery 23 and a switch 33 and being connected to the primary3| of a transformer, the secondary 32 of which is connected by atransmission line 33 with the input of the recorder 2|.

The winding of the may I3 is included in the plate circuit of agrid-controlled vapor discharge tube rectifier 34, the grid circuit ofwhich is inductively connected through a transformer 33 to the output ofthe amplifier 20. A battery 36 provides bias for the grid of tube 34. Aswitch 31 is included in the output circuit of the rectifier 34 and abattery 33 provides the plate voltage therefor.

The adjustment of the rectifier tube 3| is such that normally no currentflows in its plate circuit. The grid bias of the rectifier is adjustedto a value slightly below that required to cause an arc to strikebetween the cathode and the plate.

In the operation of the system above described for producing a waverecord, the switch I1 is closed with the relay l8 contacts and theswitch 3! closed to complete the output circuit of the rectifier 34through the winding of the relay l6 and the switches'23 and 30 likewiseare closed with the potential of the grid of the rectifier 21 soadjusted that no current flows in the plate circuit. The explosivecharge is then detonated. thereby breaking the loop 22 and causing thecessation of current flowing from the battery 24 through the transformerprimary 25. This produces a voltage surge in the transformer secondary26 which surge excites the rectifier 21 and causes current to flow inthe line 28. A fiow of current is'produced in the line 33 through theintermediary of the transformer primary 3 l' and secondary 32, therebycausing the recorder 2| to make an indication of the time of firing theshot. This time-break impulse is of insufficient magnitude to strike anarc between the filament and plate of the rectifier 34 due to theadjustment of the biasing battery 38 of said rectifier. The arrival ofthe first direct. wave at the seismometer ill produces an impulse in theamplifier 20 output which efiects a sufilcient increasein the voltageapplied to the grid 0 the rectifier 34 to cause an arc to strike betweenthe filament and plate thereof with consequent current flow in its platecircuit. Such current fiow energizes the relay l8, thereby actuating themovable contact thereof to open circuit the battery and permit thecondenser l8 to discharge as previously described, thus effectingprogressive amplification of the received waves.

After the desired waves have been received'and recorded, the switch 31is opened to de-energize the relay l3 and permit its contacts to close.Also, the various other switches are opened preparatory to conditioningthe system for its next use.

It is of course understood that modifications 'may be made in theapparatus above described without in any way departing from the spiritof the invention as defined in the appended claims.

I claim: 1. Apparatus for seismic surveying comprising a seismometer forconverting seismic waves into electrical waves, an amplifier for saidelectrical waves, a recorder for said amplified electrical waves, andmeans actuated by the arrival of said .seismic waves at said seismometerfor increasing the degree of amplification of said electrical waves. a

-2. Apparatusfor seismic surveying comprising a seismomeier forconverting seismic waves into electrical waves. an amplifier for saidelectrical waves. a recorder for said amplified electrical waves, andmeans actuated by the arrival of said seismic waves at said seismometerfor increasing the recorder response.

3. Apparatus for seismic surveying comprising a seismometer forconverting seismic waves into electrical waves. an amplifier for saidelectrical waves, a recorder for said amplified electrical waves,normally inoperative mean for increasing the degree of amplification ofsaid electrical waves, a switch for rendering operative said means, alock-in relay connected to the output of said amplifier, and meansresponsive to the actuatlon of said lock-in relay for actuating saidswitch.

4. Apparatus for seismic surveying comprising a seismometer forconverting seismic waves into electrical waves, an amplifier for saidelectrical waves, a recorder for said amplified electrical waves,normally inoperative means for increasing the degree or amplification ofsaid electrical waves, a switch for rendering operative said means, agrid-controlled vapor discharge tube rectifier. means connecting theinput of said vapor discharge tube rectifier to the output of saidamplifier, and means responsive to variation in the output of saidrectifier for actuating said switch.

5. Apparatus for seismic surveying comprising a seismometer forconverting seismic waves into electrical waves, an amplifier for saidelectrical waves, a, recorder for said amplified electrical waves,normally inoperative means for increasing the response of said recorder,a switch for rendering operative said means, a lock-in relay connectedto the output of said amplifier, and means responsive to the actuationof said lock-in relay for actuating said switch.

6. Apparatus for seismic surveying comprising a seismometer forconverting seismic waves into electrical waves, an amplifier 'for saidelectrical waves, a recorder for said amplified electrical waves,normally inoperative means for increasing the response of said recorder,a switch for rendering operative said means, a grid-controlled vapordischarge tube rectifier, means connecting the input of said vapordischarge tube rectifier to the output of said amplifier, and meansresponsive to variation in the output of said rectifier for actuatingsaid switch.

7. Apparatus for seismic surveying comprising means for convertingseismic waves into electrical waves, a variable response recorder forsaid electrical waves, and means actuated by the arrival of seismicwaves at said converting means for increasing the recorder response.

8. Apparatus for seismic surveying comprising means for convertingseismic waves into electrical waves, a variable response recorder forsaid electrical waves, normally inoperative means for increasing theresponse of said recorder, a switch for rendering operative said means,a lock-in relay connected to the output of said converting means, andmeans responsive to the actuation of said lock-in relay for actuatingsaid switch.

9. Apparatus for seismic surveying comprising means for convertingseismic waves into electrical waves, a variable response recorder forsaid electrical waves, normally inoperative means for increasing theresponse of said recorder, a switch for rendering operative said means,a grid controlled vapor discharge tube rectifier, means connecting theinput of said vapor discharge tube rectifier to the output of saidconverting means, and mean responsive to variation in the output of saidrectifier for actuating said switch.

10. A system for recording artificial seismic waves of varying amplitudeon a record strip of limited uniform width comprising a vacuum tubeamplifier having a cathode and a control electrode, an electricalseismometer, means for impressing the output voltage of said seismometerbetween said cathode and control electrode,

means for applying negative voltage to said control electrode, meanseffective at the instant of first wave reception for progressivelydecreasing the negative voltage applied to said control electrode duringsubstantially the entirev period of reception of' the seismic waves, arecorder, and connections for impressing the output of said amplifierupon said recorder.

11. Seismic prospecting apparatus comprising means for receiving seismicwaves the amplitudes of which decrease as a function of time andconverting said waves into electrical pulsations of correspondingamplitudes, a recorder connected to said means adapted to receive saidelectrical pulsations andrespond thereto in proportion to theiramplitudes, controlling means connected to said recorder for increasingthe responsiveness thereof to said pulsations as the' magnitude of thelatter decrease, and means associated with said receiving means and saidcontrolling means for setting said controlling means into operation atthe beginning of the electrical pulsations.

12;- Electronic apparatus of the character ,pe-

anlamplifier having a gain variable through a scribed adapted tomaintain a substantially con- 1 stant output when the mean signal inputthereto varies substantially unidirectionally through a wide rangeduring an interval of time, including:

range substantially equivalent to that of said input variation; meansfor automatically varying means varying said gain inversely to saidinput variation and, at least as to a portion of the variation of saidgain, independently of said signal; and means for rendering saidautomatically varying means operable substantially simultaneously withinitiation of said input variation.

13; An electronic amplifier of the character described adapted tomaintain a substantially constant output when the mean signal inputthereto varies substantially unidirectionally through a wide rangeduring a short interval of time, including: amplifying means includingat least one vacuum tube having a variable amplification factor; asource adapted to supply a bias voltage; means for impressing said biasvoltage on an element of said tube to control the amplification thereof;and means for automatically varying said voltage always through apredetermined range during said interval, said means for automaticallyvarying the voltage comprising a condenser and a resistor through whichit is adapted to discharge in said interval, means for chargingsaidcondenser to the desired initial voltage andmeans initiating dischargeof said condenser upon initiation of said signal input variation, saidvoltage variation being such as to effect a variation in saidamplification factor inverse to the variation of the signal input.

14. An electronic amplifier of the character described adapted tomaintain a substantially constant output when the mean signal inputthereto varies substantially unidirectionally through a wide rangeduring a short interval of time, including: amplifying means includingat least one vacuum tube having a variable amplification factor; acondenser adapted to have a bias voltage thereacross, means forimpressing said biasvoltage on an element of said tube to control theamplification thereof; means formaintaining the voltage across saidcondenser at a desired initial value; a discharge circuit for saidcondenser, said circuit including a resistor of such a value that thedischarge time of said condenser is substantially the same as saidinterval; and means rendering said voltage maintaining means inoperativeupon initiation of said signal input variation.

15. A method of seismic surveying which comprises receiving artificialseismic waves reflected from a plurality of sub-surface geologicalstrata of varying depths, converting the received seismic waves intoelectrical waves, amplifying and recording said electrical waves,initiating an amplification increase at a time subsequent to the time ofgeneration of said seismic waves,'and gradually and continuouslyincreasing the amplification of said electrical waves in a selecteddefinite predetermined time relation, beginning at the time of saidinitiation and extending over the time interval duringwhich seismicwaves reflected from said plurality of sub-surface geological strata ofvarying depths and compositions are being received.

18. The method of seismic surveying which comprises receiving artificialseismic waves, converting received seismic waves into electrical waves,amplifying and recording said electrical waves, initiating amplificationincrease at the time of receipt of the first seismic wave and Braduallyand continuously increasing the amplification of said electrical wavesin a selected definite predetermined time relation beginning at the 7 7time of said initiationand extending over the said gain through itsrange in said interval, said 1 time interval during which seismic wavesreflected from one or more sub-surface geological strata are beingreceived.-

WILLIAM '1. BORN.

